A Two-Phase Scenario for Bulge Assembly in Λcdm Cosmologies

Abstract

We analyze and compare the bulges of a sample of L * spiral galaxies in hydrodynamical simulations in a cosmological context, using two different codes, P-DEVA and GASOLINE. The codes regulate star formation in very different ways, with P-DEVA simulations inputing low star formation efficiency under the assumption that feedback occurs on subgrid scales, while the GASOLINE simulations have feedback which drives large scale outflows. In all cases, the marked knee-shape in mass aggregation tracks, corresponding to the transition from an early phase of rapid mass assembly to a later slower one, separates the properties of two populations within the simulated bulges. The bulges analyzed show an important early starburst resulting from the collapse-like fast phase of mass assembly, followed by a second phase with lower star formation, driven by a variety of processes such as disk instabilities and/or mergers. Classifying bulge stellar particles identified at z = 0 into old and young according to these two phases, we found bulge stellar sub-populations with distinct kinematics, shapes, stellar ages and metal contents. The young components are more oblate, generally smaller, more rotationally supported, with higher metallicity and less alpha-element enhanced than the old ones. These results are consistent with the current observational status of bulges, and provide an explanation for some apparently paradoxical observations, such as bulge rejuvenation and metal-content gradients observed. Our results suggest that bulges of L * galaxies will generically have two bulge populations which can be likened to classical and pseudo-bulges, with differences being in the relative proportions of the two, which may vary due to galaxy mass and specific mass accretion and merger histories.

An Observational Guide to Identifying Pseudobulges and Classical Bulges in Disc Galaxies

“…At present, this is no longer an ubiquitously accepted theory, rather it is thought by many that some mixture of turbulent clump instabilites early on and secular evolution in more recent epochs combine to generate many bulge propoerties (e.g. Elmegreen et al, 2008;Genzel et al, 2008;Obreja et al, 2013).…”

Relative growth of black holes and the stellar components of galaxies

“…3). The stars formed through the disc instability mode are assigned to a pseudo-bulge component, according to a widely adopted view (see, e.g., Combes et al 1990;Raha et al 1991;Debattista et al 2006;Kormendy & Kennicutt 2004;Athanassoula et al 2005; see also Mendez-Abreu 2014; Obreja et al 2013) also implemented in other SAMs (see e.g. Fontanot et al 2015a).…”

What the Milky Way bulge reveals about the initial metallicity gradients in the disc

“…Calura et al 2012;Obreja et al 2013) and the origin of its various stellar populations, as revealed by its broad metallicity distribution function (MDF;McWilliam & Rich 1994;Hill et al 2011;Ness et al 2013;Rojas-Arriagada et al 2014;Zoccali et al 2017).…”